<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>Document</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }

    div {
      width: 300px;
      height: 300px;
      background-color: red;
    }

    .box :first-child {
      float: left;
      background-color: skyblue;
    }

    .box :last-child {
      background-color: pink;
    }
  </style>
</head>

<body>
  <div class="box">
    <div></div>
    <div></div>
    <div></div>
  </div>
</body>

</html>